1. Field of the Invention
The invention relates to the purification of gases in a pressure swing adsorption system. More particularly, it relates to the improvement of product gas recovery in such a system.
2. Description of the Prior Art
The pressure swing adsorption (PSA) process provides a highly desirable means for separating and purifying at least one gas component from a feed gas mixture of said gas component and at least one selectively adsorbable component. Adsorption occurs in an adsorbent bed at a higher adsorption pressure, with the selectively adsorbable component thereafter being desorbed by pressure reduction to a lower desorption pressure. The PSA process is commonly employed in multi-bed systems. The Wagner patent, U.S. Pat. No. 3,430,418, discloses a PSA process and system employing at least four adsorption beds arranged for carrying out the PSA processing sequence on a cyclic basis. This sequence includes higher pressure adsorption, cocurrent depressurization to intermediate pressure with release of void space gas from the product end of the bed, countercurrent depressurization to a lower desorption pressure, and repressurization to the higher adsorption pressure. Wagner teaches the passing of the released void space gas from one bed directly to another bed initially at its lower desorption pressure. The pressure in the two beds is thereby equalized at an intermediate pressure, after which additional void space gas is released from the one bed as it is depressurized to a lower pressure. The other bed is further repressurized from the intermediate pressure to its higher adsorption pressure at least in part by the countercurrent addition of product effluent to the product end of the bed being repressurized.
In a further development of the art, the Fuderer patent, U.S. Pat. No. 3,986,849, discloses the use of at least seven adsorbent beds, with the feed gas mixture being introduced to the feed end of at least two adsorbent beds, in overlapping identical processing cycles, at all stages of the PSA processing sequence. It is known in the art that advantages can be achieved in particular embodiments by employing a second pressure equalization step in addition to that referred to above. By such a step, a bed undergoing repressurization is further pressure equalized with the void space gas from another bed to a higher intermediate pressure subsequent to the pressure equalization of the bed from its initial desorption pressure to an initial intermediate pressure. In accordance with the Fuderer teaching, each bed, in turn, undergoes three pressure equalization steps prior to final repressurization to the higher adsorption pressure. Fuderer also discloses the carrying out of the three pressure equalization steps in a particular manner to achieve higher product purity by substantially avoiding the impurity profile reversion that can occur upon pressure equalization between the beds, as discussed in the patent. It is also within the contemplation of the art to employ, in some circumstances, a fourth pressure equalization step prior to final repressurization with product effluent.
The PSA process is generally carried in multibed systems so as to provide a substantially uniform flow of product effluent from the adsorption system. A portion of the product effluent withdrawn from a bed or beds undergoing adsorption at higher pressure is employed for repressurization of other beds of the system. As is taught by Wagner, the last part of the repressurization is performed only with the product effluent. To avoid any discontinuity in the substantially uniform flow of product effluent in the product manifold leading from the overall PSA system, it is conventional practice to continually withdraw a portion of the product effluent for repressurization purposes.
As a result of the repressurization of the bed initially at lower pressure simultaneously by both product effluent gas and void space gas released from another bed, it has been recognized that a somewhat lower recovery of product is achieved than would result in the event such simultaneous repressurization were not required in practical commercial operations. This effect becomes more pronounced at the appreciably higher repressurization rates applicable as the number of beds in the PSA system is increased, resulting in an increase in the amount of product gas unrecovered from each bed and subsequently lost upon countercurrent depressurization and purge. Such a loss of product gas, while tolerable in light of the overall requirements of commercial embodiments of the PSA process and system, is nevertheless undesired, and its avoidance would present a further advance in the art.
It is an object of the invention, therefore, to provide an improved PSA process and system.
It is another object of the invention to provide a PSA process and system having improved product recovery.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.